The Prius's Not So Secret Gas-Mileage Secrets

Drivers everywhere recognize Toyota's Prius as the vehicle with the best fuel economy in America. How does it achieve that mileage, though? On its corporate blog, Toyota recently explained how the Prius' Hybrid Synergy Drive uses what's called an Atkinson-cycle engine to help the Prius' batteries and electric motors produce world-class gas mileage.

Employing the rarely-used Atkinson cycle twist to the four-stroke gasoline engine makes lots of sense for a gas/electric hybrid. According to Toyota, the Atkinson cycle shifts the traditional four-cycle engine's method of intake, compression, power, and exhaust. The Atkinson cycle has a longer power stroke than compression stroke, which enables the engine to produce more power in a certain operating range. The 12- to 14-percent gain in efficiency versus an Otto-cycle engine is mainly due to the reduction in pumping losses, or the amount of energy needed simply to suck air into the cylinders and then expel spent exhaust gases.

Toyota's use of a electronic continuously variable transmission--it uses a planetary gear set to vary the power split among the engine, primary, and secondary electric motors, instead of a fixed set of gears-- allows the engine to stay in its high-revving efficiency sweet spot for greater amounts of time than it would with a traditional, geared transmission. Additionally, the electric electronic motors provide the bulk of propulsion from rest, because they generate maximum torque from 0 rpm.

As mentioned, Ford's Escape Hybrid presumably uses the Atkinson cycle in its new-for-2009 2.5-liter four as it did in its 2.3-liter four cylinder powerplant used in 2008.

Toyota figures that plenty of Prius purchasers "probably don’t care about [this], and are just happy that the Prius is rated by the EPA at 48 city/45 highway mpg." But as hybrid aficionados, we appreciate being reminded about the finer points of gas/electric hybrid operation and appreciate being able to talk the talk when we boast about our remarkably fuel-efficient vehicles.